1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell having a positive electrode comprising a mixed phase metal oxide (MPO). Preferred mixed phase metal oxides include silver vanadium oxide, copper vanadium oxide and copper silver vanadium oxide present in various phases or stoichiometric formulations along with a minor amount of the starting materials.
2. Prior Art
Mixed metal oxides such as silver vanadium oxide are known for use as cathode active materials in electrochemical cells. U.S. Pat. No. 4,391,729 to Liang et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes a method of preparing a composite cathode material by thermally decomposing a vanadium salt such as ammonium metavanadate to produce vanadium pentoxide. The nitrate or the nitrite of a second metal is then added to the vanadium pentoxide, thoroughly mixed therewith and heated to dryness. At the end of the initial drying period, the mixture is again stirred and ground to ensure homogeneity and subsequently baked for a minimum of 24 hours at 360xc2x0 C. During the final heating/decomposition period, nitrogen oxide gases are evolved, and at specific time intervals after evolution of the nitrogen oxides, the admixture is stirred vigorously. The second metal is preferably selected from silver, copper, manganese and mixtures thereof. A typical product has the general formula AgxV2Oy wherein xe2x80x9cxxe2x80x9d is in the range from about 0.5 to about 2.0, and xe2x80x9cyxe2x80x9dis in the range from about 4.5 to about 6.0.
U.S. Pat. No. 5,516,340 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes the preparation of a metal oxide composite cathode material such as copper silver vanadium oxide using various starting materials including vanadium oxide combined with the nitrate or the nitrite of a second and a third metal. The reactants are thoroughly mixed together and then baked. The baking protocol calls for a gradual increase in the heating temperature up to a decomposition temperature accompanied by periodic stirring. A final grinding and heating step takes place at 375xc2x0 C. Further, U.S. Pat. No. 5,221,453 to Crespi describes various silver vanadium oxide preparation techniques, for example, milling powdered AgVO3 or powdered Ag2O with V2O5 and heating at 520xc2x0 C.
The metal oxide materials produced according to the techniques described by Liang et al., Takeuchi et al. and Crespi result in an electrode active material, preferably a cathode active material, that is advantageous for use in implantable medical devices such as an implantable cardiac defibrillator and the like where the battery power source may run under a light, device monitoring load for extended periods of time interrupted by high rate pulse discharge during device activation. However, during pulse discharge, the occurrence of voltage delay is an undesirable characteristic of some metal oxide materials, which may result in a shortened implantable device life. There is, therefore, the need for a metal oxide cathode material that provides all the advantages of the previously discussed metal oxide cathode materials, but which additionally exhibits increased discharge capacity and decreased voltage delay for pulse discharge applications. The present invention fulfills this need in a mixed phase metal oxide cathode material provided in a decomposition reaction wherein after the reactant starting materials or reactant constituents are initially combined into a homogeneous admixture, they are not further mixed during decomposition heating.
In lieu of preparation techniques calling for the mixing of the reactant constituents both before and during decomposition heating, the present invention is directed to mixed phase metal oxide active materials synthesized from a homogenous admixture of starting materials or reactant constituents that are not further mixed once decomposition heating begins. A homogeneous admixture is defined as a substantially identical distribution of the reactant starting materials or reactant constituents throughout the admixture prior to decomposition heating. The starting materials include both nitrate, nitrite, carbonate and ammonium salt materials mixed with at least one metal oxide. Thus, although the ratio and type of starting materials does not differ from the standard preparation of silver vanadium oxide according to the previously discussed patent to Liang et al., the resulting mixed phase metal oxide active materials have an increased ability for the intercalation and deintercalation of metal ions produced by oxidation of the anode while minimizing the detrimental effects of voltage delay.
Additionally, the present preparation techniques provide active materials with increased energy density which is an unexpected result.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and to the appended drawings.